In the processing of semiconductor substrates or wafers into integrated circuits, sputter etching is often used to remove a layer of material from the uppermost substrate surface. The process of sputter etching is generally known and utilizes ionized particles of a charged gas plasma to bombard the surface of a substrate and dislodge or "sputter" away substrate particles from the surface.
More specifically, the substrate to be etched is supported on an electrically charged support base or electrode within a vacuum-sealed processing chamber whereon the substrate develops an electrical charge or bias. A plasma gas is introduced into a discharge chamber opposite the surface of the biased substrate, and RF energy is generally inductively coupled to the gas such as through a coil so that an induced electric field is created inside the discharge chamber. That is, large current flow in the coil produces changing RF magnetic flux which penetrates into the discharge chamber. These changing RF magnetic fields result in changing electric fields in the discharge chamber. The energy from the induced electric field inside the chamber ionizes the gas particles. The ionized particles of the gas and free electrons collectively form what is referred to as a gas plasma or plasma cloud. The substrate is biased negatively to collect the positively charged particles from the plasma cloud. The positive ionized plasma particles are attracted to the negative substrate surface, bombarding the surface and dislodging material particles from the substrate to sputter "etch" a material layer from the substrate surface.
Conventionally, inductive energy sources utilized to create and maintain a plasma inside the chamber have been placed either inside the processing chamber and in the processing space surrounding the biased substrate, or have been placed around the outside of the chamber to surround the processing space. However, inductive energy sources positioned inside of the chamber proximate the substrate are subjected to undesired bombardment by plasma particles during the etch, and are subjected to the deposition of sputter-etched material particles thereon. Both conditions detrimentally affect the reliability of the source operation which detrimentally affects the reliability and uniformity of the plasma. Therefore, many inductive energy sources today are positioned externally around the processing chamber.
External inductive energy sources have usually included a solenoidal-shaped coil which is wound around the outside of the processing chamber to inductively couple energy to the plasma through the side chamber walls. The processing chambers and their side walls, therefore, are generally fabricated from a dielectric substance through which the inductive energy may pass, typically quartz. However, quartz processing chambers have a drawback in that particles of the substrate material, which are usually metal, do not readily adhere to quartz, and therefore, the etched material has a tendency to collect on, but eventually flake off the inside walls of the quartz chamber. Flaking detrimentally affects the plasma and contaminates the wafer. Therefore, it is an objective of the present invention to reduce flaking and substrate contamination during etching.
It is another objective of the present invention to produce a uniform, high-density plasma over a large area such that large substrate sizes might be processed. Plasma-aided manufacturing of ultra large scale integrated (ULSI) circuits requires a dense uniform plasma over large substrates having diameters of approximately 200 mm. Existing processing chambers and plasma energy sources do not adequately address such requirements and are not able to produce dense uniform plasmas over large areas.
Some sputter etching processes commonly occur at substrate voltages in the range of approximately 1,000 volts (1 kV). However, this relatively high voltage range is inappropriate for today's state-of-the-art microelectronic devices which have circuit and device features with dimensions of approximately 0.25 microns and are more susceptible to surface damage at high wafer charging voltages. As a result, lower wafer voltages, below 500 Volts, are more desirable, and preferably, voltages lower than 100 Volts are desirable, However, for an effective etch at such low voltages, a reliable, efficient and high uniform density plasma is required. Therefore, it is another objective of the present invention sputter etch substrates with small device features at low voltages without reducing the quality of the etch.
A still further objective of the present invention is to provide a sputter etch chamber and plasma source which are efficient, reliable and easy to repair and maintain. It is also an objective of the invention to produce dense uniform plasmas for a uniform etch rate at low pressures in the range of approximately 1 mTorr.